Reversing black and white scanned images

I collect legal documents from court houses around Texas (it's my job). Back when microfilm was invented, most court houses started storing their documents in a white on black (negative) format. Since then they have gone back to normal black ink on white paper format. However, many of the documents I collect are negative images.

I am working with a reinstallation of Acrobat 7.0 Pro on a new computer. Not only that but I have been working in a county with no reversed images for several months. The problem is that I have forgotten how I used to reverse the negative images I have scanned. I scan in large batches and do not stop to reverse the image during the scan. I have always made the change in Acrobat after the scan. Can someone help me remember how I did that?